1. Field of the Invention
The present invention relates to a surface acoustic wave element, an electric signal processing apparatus using the surface acoustic wave element, an environment evaluating apparatus using the electric signal processing apparatus, and an analyzing method which analyzes a material to be inspected by using the surface acoustic wave element.
2. Description of the Related Art
A surface acoustic wave element has been conventionally well known as an element which generates a surface acoustic wave on a base member, and which receives the surface acoustic wave generated on the base member.
In the conventional surface acoustic wave element, a pair of comb-shaped electrodes is provided on a flat base member. The base member is made of a piezoelectric material or a piezoelectric member is provided between the comb-shaped electrode and the base member. And, a surface acoustic wave is excited by applying a high frequency voltage to one of the comb-shaped electrodes, in the direction in which the electrode pieces of the comb-shaped electrode are arranged. Another comb-shaped electrode is disposed in the direction of propagation of the surface acoustic wave, and receives the surface acoustic wave.
The surface acoustic wave element is used for a delay line, an oscillating element or a resonating element for a transmitter, a filter for selecting frequencies, a chemical sensor, a biosensor, a remote tag, or the like.
In order to improve the performance of such a surface acoustic wave element, it is required to reduce the propagation loss as much as possible when the surface acoustic wave propagates between two pairs of comb-shaped electrodes.
However, in the usual surface acoustic wave element, since the surface of the piezoelectric material member or the surface of the base member, on either one of which the pair of comb-shaped electrodes are provided, is flat, the surface acoustic wave excited by one of the comb-shaped electrodes is diffused in a direction perpendicular to the propagating direction of the surface acoustic wave on the above-described flat surface while the surface acoustic wave is propagating toward another comb-shaped electrode, and is weakened. Therefore, the propagation loss of the surface acoustic wave cannot be reduced. This means that, in a case where the change in time passed from the transmission to the receipt of the surface acoustic wave is observed as a measuring object and is used for something, it is difficult to obtain a sufficient result from the observation because the propagation loss becomes larger as the propagation distance becomes longer. Due to these problems, the performance of the surface acoustic wave element can not be made to high.
In order to solve such problems, use of a spherical base member has been considered. When a surface acoustic wave is excited on the spherical surface of the base member, the surface acoustic wave is not diffused, and propagates around the base member a large number of times. This realizes a long propagation of the surface acoustic wave with no diffuse.
However, when the spherical base member is used, it is necessary to form comb-shaped electrodes or the like in order to apply an electric field on the surface of the base member. And, a photolithography method must be used to form a pattern which is less than 0.5 mm. This method increases the number of steps in a manufacturing process for spherical surface acoustic wave element, and increases the manufacturing cost thereof.
Further, when the electrodes made of such as metal or the like are directly formed on the surface of the base member, the surface acoustic wave which propagates around the surface of the base member is reflected by the electrodes, so that the intensity of the surface acoustic wave is rapidly reduced as the number of the round of the wave increases. Therefore, even if a sufficient and accurate evaluation needs to carry out a measurement of the time passed while the wave propagates around the base member 30 times, the surface acoustic wave is attenuated or diffused while the wave propagates around the base member about 20 times, and the evaluation cannot be carried out sufficiently and accurately.
Further, when the base member is not made of a piezoelectric material, it is necessary to provide a film of a piezoelectric material on the surface of the base member. However, since the propagation characteristic of the surface acoustic wave varies upon the thickness of this film, it is difficult to manufacture a large number of the elements. Moreover, when the base member is made of a piezoelectric material, a single crystal such as quarts, LiNbO3, LiTaO3, or the like is appropriate as a fine piezoelectric material. However, it has been difficult to realize the base member made of the single crystal because the propagation velocity of the surface acoustic wave changes while the wave propagates on the surface of the single crystal and the surface acoustic wave can not propagate around thereon.
An object of the present invention is to provide a surface acoustic wave element which can realize an extremely large number of propagation of the surface acoustic wave around a spherical base member and can carry out highly accurate signal processing and its evaluation. Another object of the present invention is to provide a surface acoustic wave element which uses a base member made of a piezoelectric material so that a surface acoustic wave can efficiently propagate on the surface of the base member. Further object of the present invention is to provide an electric signal processing apparatus using the surface acoustic wave element, and to provide an environment evaluating apparatus using the electric signal processing apparatus. More further object of the present invention is to provide an analyzing method in which a material to be inspected is analyzed by using the surface acoustic wave element.